Journal of Ceramic Processing Research. Vol. 13, No. 2, pp. 123~126 (2012)

123

J O U R N A L O F

CeramicProcessing Research

Influence of pH on the characteristics of cobalt ferrite powder prepared by a

combination of sol-gel auto-combustion and ultrasonic irradiation techniques

S. S. Madani*, G. Mahmoudzadeh and S. Abedini Khorrami

Chemistry Department, North Tehran Branch, Azad University, Tehran, Iran

Cobalt ferrite (CoFe2O4) powders with nanocrystalline sizes were produced by a combination of sol-gel auto-combustion and

ultrasonic irradiation methods employing a mixture of urea, thiourea and glycine as the fuel with the corresponding metalnitrates. The pH in the starting solution affects the combustion process, and then determines the particle size of the as-

synthesized powder. The influence of the pH value on the gel auto-combustion and the phase composition of the synthesizedpowders have been studied with the help of scanning electron microscopy (SEM) observations, Fourier transform infra red

(FTIR) spectroscopy and X-Ray diffraction (XRD) techniques. The synthesized powders had a particle size distribution in therange of 23-43 nm.

Key words: Cobalt ferrite, Nanocrystalline, Sol-gel auto-combustion, Ultrasonic irradiation, pH value.

Introduction

Magnetic nanoparticles offer great potential application

in a variety of biomedical fields, such as improved contrast

agents for magnetic resonance imaging (MRI) [1], cell

separation [2], hyperthermia tumor treatment [3] and as

magnetic field-guided carriers for localizing drugs or radio-

active therapies [4]. A ferrite with a spinel structure which

is formed by a nearly close packed fcc array of anions

with holes partly filled by the cations can be represented

by the formula AB2O4, where A represents metallic ions

located in A interstitial (tetrahedral) sites and B metallic

ions located in B (octahedral) sites. Due to the large

electronegativity of oxygen, the ionic type of bonds prevails

in almost all oxide spinels [5]. The exchange interaction

in spinel ferrites in which the antiparallel alignment of

magnetic moments of the A-site with B-site is mediated

by oxygen ions is called super-exchange interaction. The

strength of the super-exchange interaction between the

cations depends on the A-O-B bond angle, which is the

largest for an angle of 180o [6].

It is worth mentioning that, most of the magnetic

properties of CoFe2O4 strongly depend on the size and the

shape of the nanoparticles, which are closely related to

the preparation method. Various preparation techniques

have been accordingly developed to produce CoFe2O4

nanoparticles including chemical co-precipitation [7], micro-

emulsion [8], sol-gel method, sol-gel auto-combustion

[9], hydrothermal [10], solvothermal [11], organic precursor

method [12], ball milling [13], forced hydrolysis in a

polyol medium [14], mechanochemical method [15],

sonochemical [16] and complexometric synthesis [17].

Among the many synthetic methods proposed, the sol-gel

auto-combustion route, as a unique combination of the

combustion and the chemical gelation process, is preferred

in general because of its main advantages of inexpensive

precursors, short preparation time, modest heating and

relatively simple manipulations [39-41]. Although the sol-

gel route yields more promising results in the synthesis of

nanoferrites, several preparation conditions such as dilution,

fuel/oxidant ratio, pH and temperature can have an impact on

the formation of the ferrites and their properties [19].

Ultrasonic cavitation chemistry, an approach for

synthesizing a variety of compounds with milder conditions

is already the rage in materials technology. Over the last

few years, this technique has also started to catch on in

the materials science community as a way to speed

discoveries in this area. The major advantage of this new

method is that it affords a reliable and easy route for

the control of both the synthetic process and nanostructure

in advanced materials. Also, this process provides chemical

homogeneity and reactivity through atomic level mixing

within the precursor system, and phase-pure crystalline

materials can be prepared by calcining at reduced

temperatures [20].

In the present investigation, the influence of the pH

value of a mixed solution on the combustion behavior

of the precursor gel was studied.

Experimental

Materials and methodsCobalt nitrate hexa hydrate (Co(NO3)2·6H2O, Merck),

iron nitrate nona hydrate (Fe(NO3)3·9H2O, Merck), urea

(CO(NH2)2, Merck), thiourea (CS(NH2)2, Merck), glycine

*Corresponding author: Tel : +98-912-2709240Fax: +98-21-22421047E-mail: se_madani@yahoo.com

124 S. S. Madani, G. Mahmoudzadeh and S. Abedini Khorrami

(C2H5NO2, Merck) and NH4OH (Merck) were obtained of

analytical grade. All materials were used without

further purification. Deionized water was used for all the

experiments.

CoFe2O4 powders were synthesized from sol-gel auto

combustion by ultrasonic irradiation (HF-Frequenz 35 kHz,

240 W/ Made in Germany) methods. The phase identification

of the burnt powder of CoFe2O4 was done using XRD

(Model : XPERT- MPD, Philips with operated at 40 kV

and 40 mA). The structural morphology was investigated

using SEM (Phillips XL30 with 16 kV operating voltage).

An infrared study of calcined powders was done using

an FTIR instrument (NICOLET-NEXU-870).

General procedureCobalt ferrite nanocrystalline powders with a composition

of CoFe2O4 were synthesized by a combination of sol-

gel auto-catalytic combustion and ultrasonic irradiation

techniques. A flow chart for preparing nanocrystallite

ferrite is shown in Fig. 1. The detailed process can be

described as follows. An appropriate amount of nitrates

and fuel (a mixture of urea, thiourea and glycine with a

molar ratio 1 : 1 : 1) were first dissolved into deionized

water to form a mixed solution. The molar ratio of metal

nitrates to fuel was 1 : 1.2. Ammonia was added to

change the pH value of the mixed solution from 5 to 9.

Then the mixed solution was poured into a dish and

heated at 85 oC under constant stirring to transform into a

dried gel in a N2 atmosphere. Being ignited in air at

350 oC, the dried gel burnt in a self-propagating

combustion way to form a loose powder. After auto-

combustion, the combustion product powders were

calcined at 500 oC for 3 h to form the desired phase.

After that, the product was placed in an ultrasonic

irradiation bath at 20 οC for 15 minutes. The influence

of different pH values in the starting solution on the

characteristics of the CoFe2O4 powder was studied.

Results and Discussion

The experimental observations showed that gels with a

molar ratio 1 : 1.2 (nitrates: fuel), exhibit an auto-catalytic

combustion behavior. When the gels were ignited, the

combustion process rapidly propagated forward until all

the gels were burnt out completely to form loose powders.

It was also observed that the combustion rate was

influenced significantly by the pH value of the mixed

solution. With an increase in the pH value from 5 to 9,

the combustion rate and the librated gases increased

significantly. The effects of ultrasound radiation on the

chemical reactions are due to the very high temperatures

and pressures that develop during the sonochemical

cavity collapse by acoustic cavitation.

The XRD patterns of the samples are shown in Fig. 2.

This shows clearly that a pure spinel ferrite was obtained

when the stoichiometric molar ratio of Co and Fe was

used. All the samples are single phase ferrites (CoFe2O4)

with cubic spinel structures. The particle size of the

Fig. 1. Flow chart for preparing nanocrystallite ferrite powdersby a combination of sol-gel auto-combustion and ultrasonicirradiation methods.

Fig. 2. XRD patterns of samples prepared by sol-gel auto-combustionand ultrasonic irradiation methods with different pH values.

Influence of pH on the characteristics of cobalt ferrite powder prepared by a combination of sol-gel auto-combustion... 125

synthesized cobalt ferrite samples were estimated from

X-ray peak broadening of the (3 1 1) peak using the

Scherrer equation :

(1)

where D is the particle size, λ is the wavelength of Cu

Kα, β is the full width at half maxima (FWHM) of the

diffraction peaks, and θ is the Bragg’s angle.

The crystallite size for samples prepared at pH 5, 6,

7, 8 and 9 were calculated as 39, 30, 37, 23 and 43 nm,

respectively. One of the important factors which can be

affect the sol-gel process is the pH value of the solution.

When the pH value is varied, the crystal size of nanoparticles

would be changed. The crystallite percent of CoFe2O4

phase is presented in Table 1. The influence of the pH

value on the crystallite percent of the CoFe2O4 phase is

presented in Fig. 3.

SEM images of samples prepared at pH 5-9 are shown

in Fig. 4. These SEM images show that the CoFe2O4

nanoparticles prepared by a combination of sol-gel auto-

combustion and ultrasonic irradiation methods have a

uniform, almost spherical structural morphology with a

narrow size distribution of particles. The porous feature

of agglomerates is attributed to the liberation of a large

amount of gas during the combustion. The values of particle

sizes from the SEM results are in good agreement with

the particle sizes calculated by Scherrer’s equation. Energy

dispersive X-ray microanalysis (EDAX) results show that

no impurities are observed in samples at different pH

values (Fig. 5) and EDAX results are given in Table 2.

Information on the chemical changes taking place

D0.9λ

β θcos--------------=

Fig. 3. Influence of the pH value on the crystallite percent.

Fig. 4. SEM images of samples prepared at different pH values, (a)pH = 5, (b) pH = 6, (c) pH = 7, (d) pH = 8 and (e) pH = 9.

Table 1. Crystallite percent of the CoFe2O4 phase

pH 5 6 7 8 9

Crystallitepercent ofCoFe2O4

43.88 49.89 67.75 44.39 40.48

Fig. 5. EDAX analysis of samples prepared at different pH values.

126 S. S. Madani, G. Mahmoudzadeh and S. Abedini Khorrami

during the combustion process was obtained by infrared

spectral analysis, which would be very helpful for under-

standing the combustion-related reaction. Fig. 6 shows

the FTIR spectra of the CoFe2O4 powders at pH 5-9.

The scale of wave number is from 400 to 4000 cm−1. Data

analysis on the FTIR spectra of ferrite powders indicates

that the characteristic band peaking at about 570 cm−1

originates from absorption corresponding to the stretching

vibrating band of M-O (M = Fe, Co). The magnetic

properties would be affected by the external molecular

force which can lead to little shift in the vibration band.

FTIR spectra results indicate that band peaking at about

570 cm−1 confirm the XRD results. The dependence of

the band peaking area at about 570 cm−1 with pH values

is shown in Fig. 7.

Conclusions

Cobalt ferrite (CoFe2O4) powders with nanocrystalline

sizes were produced by a combination of sol-gel auto-

combustion and ultrasonic irradiation techniques employing

a mixture of urea, thiourea and glycine as the fuel (the

molar ratio of metal nitrates to fuel was 1 : 1.2). The pH

values were changed from 5 to 9. With an increase in the

pH value, the combustion rate is increased significantly.

All the samples are single phase ferrites (CoFe2O4) with

cubic spinel structures. The synthesized powders had a

particle size distribution in the range of 23-43 nm. The

XRD results show that crystallite percent of CoFe2O4 phase

grows with an increase in the pH from 5 to 7 and then it

decreases. The maximum percent of cobalt ferrite crystallites

is observed at pH = 7. FTIR spectra results indicate that

band peaking at about 570 cm−1 is high at pH values 5,

9, then it reduces at pH 6, 8 and at last this peak has

the smallest size at pH = 7.

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Table 2. EDAX results of nanocrystallites CoFe2O4 at differentpH values

pH 5 6 7 8 9

Fe2O3

(Wt %)47.92 50.05 55.05 50.99 52.21

CoO(Wt %)

52.08 49.95 44.95 49.01 47.79

Fig. 6. IR spectra of the CoFe2O4 powders at pH 5-9.

Fig. 7. The dependence of the band peaking area at about 570 cm1

with pH values.